The answer is the hub is missing the ip nhrp redirect command. In a DMVPN Phase 2 network, spoke-to-spoke tunnels rely on the hub to send NHRP redirect messages, which act as a trigger for spokes to initiate a direct shortcut. Without this command, the hub forwards traffic between spokes but never informs them of a more direct path, so even though Spoke1 and Spoke2 see each other in the NHRP cache, they never attempt to build a spoke-to-spoke GRE tunnel. This is a classic CCNP ENARSI 300-410 trap: candidates often check spoke configurations first, but the root cause lies on the hub. The exam tests your understanding that Phase 2 requires the hub to actively redirect traffic, unlike Phase 3 where spokes proactively request shortcuts. A simple memory tip is “No redirect, no shortcut”—if the hub lacks ip nhrp redirect, spokes will never initiate a direct tunnel, regardless of their cache entries.
300-410 IPv4 Access Control Lists Practice Question
This 300-410 practice question tests your understanding of ipv4 access control lists. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
DMVPN phase 2 network: spoke-to-spoke tunnels fail. Router Spoke1 has: tunnel source GigabitEthernet0/0, tunnel mode gre multipoint, ip nhrp network-id 1, ip nhrp nhs 10.0.0.1. Router Spoke2 shows: show dmvpn includes Spoke1 in NHRP cache but no spoke-to-spoke tunnel. What is the root cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The hub is not configured with ip nhrp redirect, preventing shortcut initiation.
In a DMVPN Phase 2 network, spoke-to-spoke tunnels require the hub to send NHRP redirect messages to inform spokes of the shortcut path. Without the 'ip nhrp redirect' command on the hub, spokes will not attempt to build direct tunnels, even if they have each other's NHRP cache entries. This is the root cause because Spoke1 and Spoke2 can see each other in the NHRP cache but lack the trigger to initiate the spoke-to-spoke GRE tunnel.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Spoke1 and Spoke2 have different NHRP network IDs.
The hub is not configured with ip nhrp redirect, preventing shortcut initiation.
Why this is correct
Without redirect, spokes do not learn each other's real IPs for direct tunnels.
Related concept
Read the scenario before looking for a memorised answer.
✗
Spoke1 has a firewall blocking GRE traffic.
Why it's wrong here
Firewall would block all traffic, not just spoke-to-spoke.
✗
The tunnel mode should be point-to-point for spoke-to-spoke.
Why it's wrong here
DMVPN phase 2 uses multipoint GRE.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between NHRP cache entries (which indicate registration and mapping) and the actual tunnel establishment, which requires additional mechanisms like NHRP redirect in Phase 2 or NHRP shortcut switching in Phase 3.
Detailed technical explanation
How to think about this question
DMVPN Phase 2 uses NHRP redirect (RFC 2332) as a trigger for shortcut path creation. When the hub forwards traffic between spokes, it sends an NHRP Redirect message to the source spoke, which then sends an NHRP Resolution Request to the destination spoke to establish a direct tunnel. Without 'ip nhrp redirect' on the hub, the hub forwards traffic normally but never signals the spokes to build a shortcut, so all inter-spoke traffic continues to traverse the hub, even though NHRP cache entries exist.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
IPv4 Access Control Lists — This question tests IPv4 Access Control Lists — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The hub is not configured with ip nhrp redirect, preventing shortcut initiation. — In a DMVPN Phase 2 network, spoke-to-spoke tunnels require the hub to send NHRP redirect messages to inform spokes of the shortcut path. Without the 'ip nhrp redirect' command on the hub, spokes will not attempt to build direct tunnels, even if they have each other's NHRP cache entries. This is the root cause because Spoke1 and Spoke2 can see each other in the NHRP cache but lack the trigger to initiate the spoke-to-spoke GRE tunnel.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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